The present invention generally relates to noise reduction circuits for a color video signal, and more particularly to a noise reduction circuit which is designed to suppress and reduce a noise component which is admixed with a color signal of a color video signal, without introducing a distortion in the waveform of the color signal.
For example, a color television signal generating apparatus comprising a camera tube which has a color-resolving striped filter provided at the front of the camera tube, is disclosed in U.S. Pat. No. 4,041,528 entitled "COLOR TELEVISION SIGNAL GENERATING APPARATUS FOR USE IN A SINGLE CAMERA TUBE" in which the assignee is the same as the assignee of the present application, and in U.S. Pat. No. 4,277,800 entitled "COLOR TELEVISION SIGNAL GENERATING APPARATUS" in which the assignee is the same as the assignee of the present application. According to these previously proposed color television signal generating apparatus, color signals are obtained by demodulating and detecting a carrier in an output signal of the camera tube, which carrier has a space frequency determined by a pitch of filter stripes in the color-resolving striped filter. In addition, a luminance signal is obtained by passing the output signal of the camera tube through a lowpass filter. The color signals and the luminance signal are subjected to a matrix operation, so as to obtain three primary color signals. The three primary color signals have a frequency band between zero and approximately 750 kHz, for example, and include a noise which is evenly distributed in the form of a white noise. The quantity of this noise is proportional to 1/2 the power of the bandwidth of the above frequency band. Accordingly, it is possible to reduce the high-frequency noise by limiting the frequency band to a narrow frequency band. That is, the noise quantity becomes 1/.sqroot.2 the original quantity when the frequency band is reduced to 1/2 the original frequency band. However, when the the frequency band is limited to the narrow band, a high-frequency signal component is also eliminated together with the noise component, and a distortion is inevitably introduced in the signal waveform. This distortion in the signal waveform deteriorates the resolution. Accordingly, it is necessary to compensate for such a distortion in the signal waveform.
The conventional noise reduction circuit was made up of a highpass filter, a lowpass filter, a low level compressing circuit, and an adder, as will be described later on in the specification in conjunction with the drawings. In the conventional noise reduction circuit, a high-frequency noise component is eliminated by limiting the band of the color signal in the lowpass filter. On the other hand, the color signal is passed through the highpass filter so as to obtain a high-frequency component signal, and this output of the highpass filter is passed through the low level compressing circuit so as to cut a low-level noise. The output signal of the low level compressing circuit and the output signal of the lowpass filter are added in the adder, so as to compensate for the distortion in the signal waveform.
However, when eliminating the low-level noise in the low level compressing circuit of the conventional noise reduction circuit, a low-level part of the signal component is also eliminated. Thus, the waveform of the signal which is obtained by adding the output color signal of the lowpass filter and the output signal of the low level compressing circuit in the adder, does not become the same as the waveform of the original color signal, and a distortion is introduced at leading and trailing edges of the color signal waveform. Consequently, there was a disadvantage in that it was impossible to carry out a correct color reproduction.